Method and device for controlling handover of user equipment in long connected mode in wireless communication system

ABSTRACT

The present invention relates to a method and a device for effectively controlling a handover of a user equipment (UE) in a long connected mode. A method for controlling a handover by an eNB, according to one embodiment of the present invention, comprises the steps of: checking whether a UE is in a long connected mode; and transmitting, to the UE, channel measurement configuration information of which a channel measurement report trigger condition is alleviated compared with a normal connected mode.

TECHNICAL FIELD

The present invention relates to a wireless communication system. Inmore particular, the present invention relates to a method and apparatusfor controlling handover of a User Equipment (UE) in long connected modein a wireless communication system.

BACKGROUND ART

Mobile communication systems were developed to provide the subscriberswith voice communication services on the move. With the rapid advance oftechnologies, the mobile communication systems have evolved to supporthigh speed data communication services beyond the early voice-orientedservices. However, the limited resource and user requirements for higherspeed services in the current mobile communication system spur theevolution to more advanced mobile communication systems.

As one of the next-generation mobile communication systems to meet suchrequirements, standardization for a Long Term Evolution-Advanced (LTE-A)system is underway in the 3^(rd) Generation Partnership Project (3GPP).LTE-A is a technology designed to provide high speed packet-basedcommunication of up to 1 Gbps.

In such a wireless communication system, the UE in the connected mode isallocated radio resource for data communication and, when the datacommunication is complete, releases the radio resource to enter the idlemode. The UE in the idle mode performs signaling to be allocated radioresource for data communication again.

Meanwhile, with the popularization of smartphone, it occurs frequentlythat multiple applications are running simultaneously. This causes theUE to generate small data such as keepalive and state transitionmessages frequently.

In this case, the UE also has to transmit signals to request forallocation and release of the radio resource and core network resourcefor data communication, resulting in signaling overload of the network.

FIG. 1 is a diagram illustrating the network architecture of an EvolvedPacket Core (EPC) network to which the present invention is applied. InFIG. 1, only the entities related to the present invention are depictedamong the entities constituting the EPC. The EPS may include otherentities responsible for supplementary functions not dealt with herein.

Referring to FIG. 1, the User Equipment (UE) 100 denotes a terminal, andevolved Node B (eNB) 102 denotes an entity of controlling radio resourceto which the UE connects through a radio channel.

The Mobility Management Entity (MME) manages the UE 100 in the idle modeand is responsible for the functions related to roaming andauthentication of the UE 100. The MME 108 also processes the bearersignal generated by the UE 100.

The Home Subscriber Server (HSS) stores UE-specific subscriberinformation which is provided to the MME 108 for use in controlling theUE 100 when the UE 100 connects to the network.

The Serving Gateway (SGW) 104 manages user the user bearer of the UE andnotifies the MME of the arrival of data addressed to the UE. The PGW 106delivers data from a service network to the UE or from the UE to theservice network. The PGW 106 has a policy for processing the data.

In the EPS, the UE establishes a radio bearer with the eNB for datacommunication, and the eNB and MME perform context setup and establishS1 connection therebetween.

If a predetermined condition is fulfilled after completing datacommunication, the MME or eNB performs eNB context release, i.e.releases the S1 connection. The eNB also performs RRC Connection Releasesuch that the UE transitions to the idle mode.

In the case that the UE communicates small data frequently, however, theUE has to transition between the connected mode and the idle modefrequently and thus the network perform S1 connection and data bearerestablishment repeatedly, resulting in increase of traffic load.

There is therefore a need of minimizing the number of state transitionsof the UE and, in order to achieve this, it is effective to make the UEstay longer in the connected mode. That is, in order to process thesmall data occurring in the state of no other data communication, thenetwork controls the UE to stay in the connected mode.

However, if the UE stay long in the connected mode, this is likely tocause handover overload.

DISCLOSURE OF INVENTION Technical Problem

The present invention has been conceived to solve the above problem. Thepresent invention aims to provide a method and apparatus for controllinghandover of the UE to reduce signaling overhead when the UE is in thelong connected mode.

That is, the present invention aims to reduce the handover signalingoverload occurring to the UE staying in the long connected mode.

Solution to Problem

In order to solve the above problem, the network of the presentinvention includes a method of controlling handover of a terminal, amethod of minimizing a number of handovers of the terminal, a method oftransitioning, when handover is required, the terminal to the idle modeother than performing handover, and a method of minimizing handoversignaling and context.

Advantageous Effects of Invention

The present invention is advantageous in terms of reducing the handoversignaling overload by handling the handover efficiently especially whenthe UE is in the long connected mode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the network architecture of an EvolvedPacket Core (EPC) network to which the present invention is applied.

FIG. 2 is a diagram illustrating a handover situation of the UEaccording to an embodiment of the present invention.

FIG. 3 is a diagram illustrating a handover situation of the UEaccording to another embodiment of the present invention.

FIG. 4 is a signal flow diagram illustrating the conventional UE's S1handover procedure.

FIG. 5 is a signal flow diagram illustrating the handover procedureaccording to the third embodiment of the present invention.

FIG. 6 is a schematic diagram of an evolved Node B (eNB) base station.

MODE FOR THE INVENTION

In the following description, the term ‘small data’ denotes thebackground data such as keepalive and state transition messages that arecharacterized by low data rate and delay tolerance.

In the following description, the term ‘long connected mode’ denotes theoperation mode in which the UE stays in the connected state withoutongoing data.

It is obvious that the present invention is not limited to thedescription set forth herein, but various modifications and changes canbe made thereto without departing from the broader spirit and scope ofthe invention. Detailed descriptions of well-known functions andstructures incorporated herein may be omitted to avoid obscuring thesubject matter of the present invention.

The same reference numbers are used throughout the drawings to refer tothe same or like parts. In the drawings, certain elements may beexaggerated or omitted or schematically depicted for clarity of theinvention. This aims to clarify the subject matter of the presentinvention by omitting unnecessary description of the functions notrelated to the present invention. Exemplary embodiments of the presentinvention are described herein with reference to the accompanyingdrawings in detail.

In order to handle the handover of the UE in the long connected mode,the present invention proposes 1) a method for the network to adjust themeasurement configuration of the UE so as to be different from that ofthe legacy connected mode or configure the measurement reporttransmitted by the UE differently of the legacy connected modeprocessing condition, 2) a method for the network to transition the UEoperation mode to the idle mode when handover is triggered, and a lighthandover method for transferring the least context without dataforwarding process in the middle of the handover procedure.

In order for the eNB to control the UE handover according to the presentinvention, it is necessary to know that the UE in the long connectedmode. The eNB may determine whether the UE is in the long connected modethrough a) by using a specific variable or part of the subscriptioninformation which is transmitted by the MME and/or by checking whetherthere is data transmitted by or to the UE in the eNB.

First Embodiment

A method is provided for minimizing the number of handovers of the UE insuch a way that the network adjusts the measurement configuration of theUE differently of the legacy mode or configures the measurement reportsent by the UE differently of the legacy connected mode processingcondition.

According to the first embodiment, the eNB, e.g., eNB 100 of FIG. 6, maymanage the measurement configuration value of the UE in the longconnected mode separately. In more detail, the eNB may send a separatechannel measurement configuration via a transceiver 110 to the UE in thelong connected mode through the RRC Connection Reconfiguration(RRCConnectionReconfiguration) message in the Attach or Service Requestprocedure.

For example, it is possible to extend the measurement report interval ofthe UE or decreases the threshold value of the measurement report of theUE as compared to the legacy connected mode. Such a value may be a newvariable value or legacy variable value modified in the MeasConfig IE ofthe RRCConnectionReconfiguration message.

The eNB also may manage the measurement report processing condition forthe UE in the long connected mode separately via a control unit 120,which can be implemented by using, for example, at least one hardwareprocessor. The eNB receives a measurement report via the transceiver 110from the UE and determines via the control unit 120 whether to triggerhandover of the UE according to the processing condition. The controlunit 120 may comprise, for example, at least one controller, at leastone processor, or the like.

According to the measurement report processing condition, the thresholdfor the eNB to make a handover decision for the UE in the long connectedmode is set loosely. For example, the eNB may set the threshold of thereference signal strength for handover of the UE in the long connectedmode to a value less than that of the normal connected mode.

In this case, the eNB may have the measurement report processingcondition for the UE in the long connected mode as described above. Inthis method, the UE maintains the connection to the corresponding celleven when the signal strength is lower than the signal strengththreshold of the normal connected mode, thereby giving the effect ofbroadening the cell coverage.

Through this method, the eNB according to the first embodiment of thepresent invention adjusts the measurement configuration so as tominimize the number of handovers of the UE in the long connected mode.

The measurement report processing condition and measurementconfiguration value for the UE in the long connected mode may be fixedcondition and value preset by the operator or variable condition andvalue set by the eNB in adaptation of the network status. The eNBs inthe network may be are all configured identically or differently. All ofthe UEs in the long connected mode are configured identically ordifferently.

FIG. 2 is a diagram illustrating a handover situation of the UEaccording to an embodiment of the present invention.

In the case of moving on the path as shown in the drawing, the UE in thenormal connected mode is handed over from the cell 1 to the cell 2 viathe cell 3 according to the handover command message from the eNB.

Meanwhile, the UE in the long connected mode proposed in the presentinvention maintains connection to the cell 1 on the move withouthandover. Since the UE in the long connected mode is in the state ofoperating at low data rate and/or communicating delay tolerant smalldata or having no ongoing data, it is possible to put up with thecommunication error caused by the interference from the eNB2 or eNB3.

If the UE does not fulfill the least condition for maintaining theconnection to the cell 1 in the long connected mode (e.g. signalstrength becomes equal to or less than the threshold value) at a certainlocation, the eNB makes a handover decision for the UE to the cell 2.

Second Embodiment

Method for the network to transition the UE operation mode to the idlemode when the UE handover decision is made

The second embodiment proposes a method of transitioning the UEoperation mode to the idle mode in the situation where the UE in thelong connected mode which has no ongoing data is handed over.

The eNB analyzes the measurement report sent by the UE in the longconnected mode. If the measurement result fulfils the condition oftriggering handover of the UE, the eNB transmits the UE operation stateto the idle mode by triggering the S1 release procedure other thanmaking any handover decision for the UE. The handover condition maycorrespond to the configuration for the UE in the long connected mode asdescribed in section 1).

FIG. 3 is a diagram illustrating a handover situation of the UEaccording to another embodiment of the present invention.

In the case of moving on the path as shown in the drawing, the UE in thenormal connected mode is handed over from the cell 1 to the cell 2according to the handover command message from the eNB 1.

In the case of the UE in the long connected mode proposed by the presentinvention, if the measurement report transmitted by the UE fulfils thecondition for handover of the UE, the eNB 1 triggers the S1 releaseprocedure other than making a handover decision for the UE to the cell2. That is, the eNB 1 transmits the RRC Connection Release message tothe UE.

Meanwhile, the configuration for the UE in the long connected modeproposed in the first embodiment may be applied to the secondembodiment. The UE in the long connected mode according to the firstembodiment maintains the connection to the current cell unlike the UE inthe normal connected mode which performs handover in such a situation.

If the UE does not fulfill the least condition for maintaining theconnection to the current cell in the long connected mode (e.g. signalstrength becomes equal to or less than the threshold value) at a certainlocation, the eNB may release the S1 connection for the UE.

Through this method, it is possible to reduce the number of transitionsto the idle mode as compared to the case operating in the normalconnected mode.

Third Embodiment

Light handover method for handover of the least context without dataforwarding process in the middle of handover procedure

According to the third embodiment, when the UE in the long connectedmode performs handover in the situation without ongoing data, thenetwork operates without data forwarding process during the legacyhandover procedure. In this way, it is possible to reduce handoversignaling overhead.

FIG. 4 shows a S1 handover procedure of the UE in the normal connectedmode. Briefly, the eNB makes a handover decision for the UE and notifiesthe source MME of this, and the source MME sends the target MME arequest for data forwarding. The source and target MMEs perform dataforwarding in cooperation with the source and target SGWs. This meansthat the data forwarding occupies large part of the handover procedure.

FIG. 5 is a signal flow diagram illustrating S1 handover of the UEaccording to an embodiment of the present invention.

1. The source eNB makes a handover decision for the UE. The source eNBknows that the UE is in the long connected mode and has no ongoing data.(step 501)

2. The source eNB sends the source MME a HO required message. The HOrequired message may include or not the information indicating that theUE is in the long connected mode depending on the method of checkingwhether the UE is in the long connected mode. (step 502)

3. The source MME sends the target MME the Forward Relocation Requestmessage to notify of no necessity of data forwarding process. Thisinformation may be carried by a new parameter or a modified value of alegacy parameter in the Forward Relocation Request message. Theinformation “Forward Relocation is not required” as shown in FIG. 4 isapplicable to both the above methods. (step 503

4. The target MME stores the information that the corresponding UE isnot in the long connected mode if necessary. The information that the UEis in the long connected mode may be used as a condition for determiningthat the omission of data bearer-related parameter in the UE contextmanaged by the MME is not error. The target MME also may omit the databearer-related parameter and sends the eNB the HO request message. (step530)

5. The target MME sends the target eNB the HO request message includingthe information indicating that the UE is not in the long connectedmode. This information may be carried by a new parameter or a modifiedvalue of a legacy parameter in the HO request message. The information“long connected” as shown in FIG. 4 is applicable to both the abovemethods. (step 505)

6. The target eNB store the information that the corresponding UE is notin the long connected mode if necessary. (step 540)

7. The subsequent procedure follows the standard procedure.

According to another embodiment, the target eNB may acquire theinformation that the corresponding UE is not in the long connected modefrom a transparent container transmitted from the source eNB to thetarget eNB. The source eNB transmits the information that thecorresponding UE is in the long connected mode using the transparentcontainer which is delivered to the target eNB via the source and targetMMEs as shown in FIG. 4.

The invention claimed is:
 1. A method for controlling handover of aterminal by a base station, the method comprising: identifying whetherthe terminal is in a first connected mode or a second connected mode;transmitting, if the terminal is in the first connected mode, to theterminal channel measurement configuration information with a firstchannel measurement report trigger condition; and transmitting if theterminal is in the second connected mode, to the terminal the channelmeasurement configuration information with a second channel measurementreport trigger condition.
 2. The method of claim 1, wherein the terminalis a connected state with ongoing data in the first connected mode, andthe terminal is the connected state without ongoing data in the secondconnected mode.
 3. The method of claim 1, wherein each of the first andthe second channel measurement report trigger condition includes atleast one of a channel measurement report trigger interval or a signalstrength threshold value.
 4. The method of claim 3, wherein the channelmeasurement report trigger interval of the second channel measurementreport trigger condition is longer than the channel measurement reporttrigger interval of the first channel measurement report triggercondition.
 5. The method of claim 3, wherein the signal strengththreshold value of the second channel measurement report triggercondition is lower than the signal strength threshold value of the firstchannel measurement report trigger condition.
 6. The method of claim 1,further comprising: receiving from the terminal a measurement reportbased on the channel measurement configuration information; if theterminal is in the first connected mode, determining whether to controlhandover of the terminal based on a first measurement report processingcondition, and if the terminal is in the second connected mode,determining whether to control handover of the terminal based on asecond measurement report processing condition.
 7. The method of claim6, wherein each of the first and the second measurement reportprocessing condition includes a signal strength threshold value, andwherein the signal strength threshold value of the second measurementreport processing condition is lower than the signal strength thresholdvalue of the first measurement report processing condition.
 8. A basestation for controlling handover of a terminal, the base stationcomprising: a transceiver configured to transmit and receive a signal toand from the terminal; and a processor configured to: identify whetherthe terminal is in a first connected mode or a second connected mode; ifthe terminal is in the first connected mode, transmit to the terminalchannel measurement configuration information with a first channelmeasurement report trigger condition; and if the terminal is in thesecond connected mode, transmit to the terminal the channel measurementconfiguration information with a second channel measurement reporttrigger condition.
 9. The base station of claim 8, wherein the terminalis a connected state with ongoing data in the first connected mode, andthe terminal is the connected state without ongoing data in the secondconnected mode.
 10. The base station of claim 8, wherein each of thefirst and the second channel measurement report trigger conditionincludes at least one of a channel measurement report trigger intervalor a signal strength threshold value.
 11. The base station of claim 10,wherein the channel measurement report trigger interval of the secondchannel measurement report trigger condition is longer than the channelmeasurement report trigger interval of the first channel measurementreport trigger condition.
 12. The base station of claim 10, wherein thesignal strength threshold value of the second channel measurement reporttrigger condition is lower than the signal strength threshold value ofthe first channel measurement report trigger condition.
 13. The basestation of claim 8, wherein the processor is further configured to:receive from the terminal a measurement report based on the channelmeasurement configuration information; if the terminal is in the firstconnected mode, determine whether to control handover of the terminalbased on a first measurement report processing condition; and if theterminal is in the second connected mode, determine whether to controlhandover of the terminal based on a second measurement report processingcondition.
 14. The base station of claim 13, wherein each of the firstand the second measurement report processing condition includes a signalstrength threshold value, and wherein the signal strength thresholdvalue of the second measurement report processing condition is lowerthan the signal strength threshold value of the first measurement reportprocessing condition.